In recent years, advances are being made in the development of a hybrid electric vehicle (HEV), an electric vehicle (EV), and also a fuel cell vehicle. With regard to these kinds of so-called electric vehicles, it is necessary to utilize a power supply unit capable of repeating charge and discharge. Examples of the power supply unit include a secondary battery such as a lithium ion battery and a nickel hydrogen battery, and an electric double layer capacitor. In particular, a lithium ion secondary battery is considered to be favorable for an electric vehicle because of high energy density and high durability against repeated charge and discharge. Thus, developments are being promoted with respect to various types of secondary batteries. It is to be noted that, in order to apply a secondary battery to a power supply for driving a motor used in the above-mentioned various types of vehicles, a plurality of secondary batteries connected in series are required to ensure high output power.
However, when batteries are connected to each other via a connector, output is decreased because of electrical resistance of the connector. In addition, the batteries including the connector are disadvantageous in a spatial aspect. In other words, the connector causes a decrease in output density or energy density in the batteries.
For the solution of such a problem, advances are being made in the development of a bipolar secondary battery such as a bipolar lithium ion secondary battery. The bipolar secondary battery has a configuration in which a plurality of bipolar electrodes, each of which is provided with a positive electrode active material layer formed on one side of a current collector and a negative electrode active material layer formed on the other side, are stacked on top of each other via an electrolyte layer or a separator.
The current collector used in the bipolar secondary battery as described above is preferably made from a material that is more lightweight and has excellent electrical conductivity in order to ensure large output density. Therefore, a current collector has been proposed in recent years, which is composed of a polymer material to which an electrically conductive material is added instead of conventional metal foil. For example, Patent Literature 1 discloses a current collector, in which metal particles or carbon particles are mixed in a polymer material as an electrically conductive material, and resin having electrical conductivity is contained.